The invention relates to a locator, in particular a handheld locator, for detecting inclusions in walls, ceilings and/or floors with the aid of a capacitive sensor device that makes it possible to detect differences in impedance of a measurement signal and draw conclusions about the hidden objects that generate the differences in impedance.
For locating inclusions in wall material, essentially two different measuring methods are used at present. The inductive methods, which are the basis for a first family of locators, utilize the fact that introducing metal objects into magnetic alternating fields affects the course of the magnetic field lines. This influence of the metal objects is expressed for instance in the amount of the impedance of a coil that generates a magnetic alternating field of this kind. Inductive methods are especially well suited for detecting ferromagnetic materials. However, the search for nonmagnetic materials, such as copper or plastic, presents difficulties. Plastic lines, especially, which are increasingly used in the field of installation work, cannot in principle be found by this method.
The three-dimensional sensitivity of a sensor that functions inductively is directly associated with the three-dimensional distribution of the magnetic fields generated by the measuring coils. It is decisive in designing a sensor to take a directional characteristic into account. When the locator is placed on an object to be investigated, such as wall material, the sensor should as much as possible detect only hidden objects inside the wall, and thus the locator should respond only to objects that are located in front of the transmitter. Objects behind and next to the sensor must not alter the outcome of measurement.
In sensors that function inductively, this problem is typically solved by using ferrite bodies onto which the coils of the sensor of the locator are wound. These ferrite bodies have the property of focusing the electromagnetic field generated by the coils and thus of guiding it in a certain way. Focusing the field can be expressed for instance in the course of the field lines and can also be detected. The parameter that is decisive for focusing the field lines and thus for generating the directional characteristic of such a sensor is the amount of magnetic susceptibility of the core material (ferrite body). With ferromagnetic materials, magnetic susceptibility values on the order of magnitude of 100 are technically not a problem and can be attained economically. If a suitable core geometry is selected, the desired directional characteristic can therefore be attained by simple means and at low cost, for inductive locating methods.
A second class of location sensors or locators utilizes capacitive methods for detecting enclosed objects. Such capacitive methods are currently used in the building trade to search for instance for substructures, studs, and comparable wall inclusions in lightweight buildings. The key element of a capacitive sensor or locator of this kind is a capacitor element. The measurement principle fundamental to this method is based on the variation in the impedance of the measuring capacitor by the dielectric medium surrounding it. The presence of an object with a deviant dielectric constant in the surroundings of the measurement sensor results in a variation in the capacitance of the sensor element and thus in an electrically measurable effect.
Unlike inductive locators, in capacitive sensors it is markedly more difficult to achieve a directional characteristic. Although here as well, analogous to the methods in inductive sensors described above, it is conceivable in principle to use materials that focus the electrical field, nevertheless at feasible costs for such a detection system to be used commercially, it is realistically possible to use only such materials as have low values of the dielectric constant ∈ and thus a low capability of focusing the electrical field, or of focusing the electric field lines described by the electrical field. Typical values of the dielectric constant E for usable materials are on the order of magnitude of 5, so that an adequate directional characteristic requires the use of more-complicated and thus more-cost-intensive focusing mechanisms and shielding geometries.
From U.S. Pat. No. 5,726,581, a capacitive proximity sensor is known, in which the current through a sensor element is increased by the presence of an object, so that this increase in current can be detected from the altered voltage drop at a resistor. To generate a certain directional characteristic for the measurement field, an additional shielding electrode is applied to the side of the sensor electrode remote from the object. Both electrodes are connected to a common ground potential.
The change in capacitance of the sensor electrode that is due to the object is maximized by providing that the capacitance between the sensor element and ground is minimized. This is attained by providing that the electrical field lines originating at the measuring electrode are deformed, over a wide three-dimensional range, by the larger shielding electrode in such a way that a direct connection with the ground potential is not possible. In this way, a certain directional effect of the electrical field of the measuring electrode is also generated.
A further problem in constructing capacitive sensors is undesired crosstalk between the conductor faces of the measuring capacitor on one side and the electronic components of the evaluation circuit on the other. If even small objects are to be detected, even the slightest signals must be filtered out of the background noise in the measurement signal. Crosstalk of the electrical fields of the measurement signal, for instance to the electronic components of the evaluation device of a locator of this kind, can alter the measurement outcomes and thus can make precision measurements impossible. For this reason, these two component parts, that is, the actual capacitive sensor element and the electronic evaluation circuits of the capacitive locator, are often disposed spatially separately from one another and are connected to one another by cables.
It is the object of the invention to disclose a capacitive locator of the type defined at the outset that has a compact structure, low cost, and easy technical feasibility along with adequate directional precision. It is also the object of the invention, in a locator of the type described at the outset, to realize parasitic crosstalk phenomena between the capacitive sensor device and electronic components for generating and evaluating the measurement signal of such a device, by means that are as simple as possible from a production standpoint yet are mechanically stable.